Field of the Invention.
The invention relates generally to downhole tools for use in wells, and more particularly to means for controlling a downhole linear motor to stop free fall of the motor's mover, which might otherwise damage the motor.
Related Art.
In the production of oil from wells, it is often necessary to use an artificial lift system to maintain the flow of oil. The artificial lift system commonly includes an electric submersible pump (ESP) that is positioned downhole in a producing region of the well. The ESP has a motor that receives electrical signals from equipment at the surface of the well. The received signals run the motor, which in turn drives a pump to lift the oil out of the well.
ESP motors commonly use rotary designs in which a rotor is coaxially positioned within a stator and rotates within the stator. The shaft of the rotor is coupled to a pump, and drives a shaft of the pump to turn impellers within the body of the pump. The impellers force the oil through the pump and out of the well. While rotary motors are typically used, it is also possible to use a linear motor. Instead of a rotor, the linear motor has a mover that moves in a linear, reciprocating motion. The mover drives a plunger-type pump to force oil out of the well.
In order to properly control a linear motor, it is desirable to know the position of the mover within the stator. Linear motors may use several sensors (e.g., Hall-effect sensors) to determine the position of the mover. The signals from these sensors (which may be referred to herein as the Hall signals) are provided to a control system, which then produces a drive signal based upon the position of the mover and provides this drive signal to the motor to run the motor. In one system, the state of the drive signal is automatically commutated based on the Hall signals. The speed of the motor is controlled by increasing or decreasing the voltage of the drive signal.
In one particular system, the power stroke of the linear motor (the stroke that drives the pump to pump fluid) is downward. There is normally a certain amount of back pressure during the power stroke which prevents the mover of the linear motor from free-falling due to gravity. This back pressure is caused by the fluid that is present in the barrel of the pump and/or the production tubing above the pump. (The fluid is drawn into the pump on the return stroke and forced from the pump to the production tubing on the power stroke.) In some situations, this fluid may not be present in the pump, such as when the pump is started, or if gas in the fluid causes the pump to lose its prime. The absence of back pressure from the fluid may allow the motor's mover to free fall under the force of gravity. Because the mover may be quite heavy, the free fall of the mover into a hard stop at the bottom of the motor may damage the motor. It should be noted that this may be a problem when the downward stroke of the motor is the return stroke, as well as the power stroke.
It would therefore be desirable to provide means to prevent the free fall of the mover, and thereby prevent damage that might result from this situation.